Acousto-Optics Detects Fluorescence

STUTTGART, Germany -- A team of physicists at the University of Stuttgart has captured time-resolved images of collapsing microbubbles in water. The researchers used a sensitive, high-speed streak camera to study the dynamics of the light and shock waves emitted by a single collapsing bubble.

The underlying effects of cavitation and sonoluminescence are well-known. Optical components, semiconductors or eyeglasses are often cleaned in an ultrasound bath where pressure waves stress and compress gas bubbles in the water. If the pressure variations are too strong, the bubbles begin to collapse and periodically expand. Under certain conditions, each collapse is accompanied by a flash of light. This effect is called sonoluminescence, which can be described as the conversion of sound into light.

Researchers used a relatively simple setup to study the dynamics of light and shock waves from a collapsing bubble. The blue spot in the center is the bubble. The piezos on the right and left of the flask generate the acoustic field; the hydrophone for measuring the driving pressure faces downward.
However, this explanation implies that enormous pressure and temperature must be present in the imploding bubble to generate photons. "The collapse of a bubble can be understood as focusing energy," explained Bruno Gompf, one of the researchers.

To obtain spatial information, Gompf and colleague Rainer Pecha trapped a single bubble, using a standing sound wave in a resonator. The stability of the bubble position combined with the optical system and the resolution of their Hamamatsu C5680 camera produced a total spatial resolution of 13 µm. On the time axis, the camera's 25-µm slit enabled 400-ps resolution. More important, the triggering matched the camera's 50-ns time window with the collapses, which occurred every 20 µs.

The preceding sonoluminescence flash was used as a trigger source fed into a delay circuit. To observe the development of the collapse and the generated shock wave, Gompf and Pecha used HeNe laser light, which was scattered by the bubble.

The images reveal the development of a spherical shock wave over time. Shortly after the collapse and simultaneous light emission, the shock wave travels almost 4000 m/s, more than three times that of the normal speed of sound in water. The pressure in the vicinity of the bubble at this moment can be calculated from this, resulting in values in the range of 60,000 atmos.

The key question is: How strong can the energy concentration be made? Theorists expect that conditions sufficient for nuclear fusion are possible. But with real bubbles, this comes down to the question of how long the shape stays spherical.

Gompf said that a perfect spherical implosion would lead to extremely high energies. Although such an extreme case would mean an idealization, scientists are keen to see when and why shape instabilities begin and what the limits are for the energy-focusing collapse.